Pin tumbler locks have been known heretofore. Such locks generally include a shell or housing in which a plurality of pin bores or chambers are provided. A cylinder or plug member is rotatable within the shell and is provided with a keyway ridge which extends axially of the cylinder within the keyhole thereof at approximately its mid-point. The pin bores are located within the shell such that pin members positioned therewithin are urged by spring means into seating engagement along the keyway ridge of the cylinder. The pin members are dimensioned such that they are out of longitudinal alignment at the shear point (the interface between the housing and cylinder) of the lock, and thus prevent rotation of the cylinder. An extension, frequently referred to as the "tail", is provided at the end of the plug. The tail is adapted, upon rotation of the plug, to turn a cam which actuates a bolt or bolt-actuating mechanism. Insertion of the correct key into the keyhole serves to lift the several pin members and seat them in the notches of the key. In this manner the pin members are aligned within the pin bores or chambers such that the upper pin segments, referred to as drivers, are positioned within the shell whereas the lower pin segments, referred to as pins, are positioned within the plug or cylinder. Thus, with all of the pins so aligned at the shear point of the cylinder it is rotatable to activate the bolt mechanism.
One of the principal drawbacks of the pin tumbler cylinder locks described above is its susceptibility to being picked. Briefly, picking such a lock involves the use of a tension wrench which is inserted into the lock and subjects the cylinder to torque. Simultaneously a pick is inserted into the lock to manipulate the pins sequentially within their chambers until the shear point for each is reached. At such time the cylinder will rotate slightly upon the freeing of the cylinder at that particular axial location. The pins in succeeding chambers are thus similarly manipulated until the shear point for the pin in each of such chambers has been found. When the shear point for the pin in the closest chamber to the exterior of the lock has been located, and the pin therein aligned with the shear point, the cylinder is completely rotatable and the bolt can be unlocked. The construction of such locks in permitting this direct interaction between the cylinder and bolting mechanism, and the detection of the alignment of the various pins at their shear points, during picking is an inherent weakness of such locks in respect of the degree of protection which they afford. The inherent weakness also stems from the fact that the lock element which is actuable directly by the key, and is accessible to the holder of the key, also operates the bolting mechanism.